The distance should be 4m from the wire in order to get the magnetic field of 0.100μ .
The magnitude and direction of the magnetic field due to a straight wire carrying current can be calculated using the previously mentioned Biot-Savart law. Let "I" be the current flowing in a straight line and "r" be the distance. Then the magnetic field produced by the wire at that particular point is given by ...(1)
Since the wire is assumed to be very long, the magnitude of the magnetic field depends on the distance of the point from the wire rather than the position along the wire.
It is given that magnetic field 40.0 cm away from a straight wire is 1.00μT having current 2.00 A .
From equation (1) magnetic field 40.0 cm = 0.4m away from a straight wire is 1.00μT which is given by .....(2)
From equation (1) magnetic field 'r' m away from a straight wire is 0.100μT which is given by ...(3)